This invention pertains generally to magnetic probes and more particularly to such transducers incorporating magnetic bridge arrangements which are operable to map ferromagnetic discontinuities within or on a specimen.
There presently exists a need for determining the buildup of corrosion products ("sludge") on the outer surfaces of steam generator tubing which will, if undetected, reduce the efficiency of the operating components. This is particularly important in pressurized water reactor electrical generating facilities where the problem is complicated by lack of direct access to the outer surfaces of the steam generator tubes when the tubes are in place. In addition, in such an application, remote monitoring is a necessity to avoid exposure to radiation. Any measuring device which is to be employed in such an application therefore must function when inserted inside the coolant tubes. In other words, such a device must be capable of sensing corrosion products when separated from these materials by the metal walls of the heat exchanger tubing. The requirement that such a probe function when separated from the material to be monitored by a continuous metallic wall is a particularly stringent one, since it rules out most of the experimental approaches which might otherwise be employed.
The corrosion products which are formed under reactor conditions are known to contain, as the major component, Fe.sub.3 O.sub.4, a strongly ferromagnetic substance. Ferromagnetism therefore suggests itself as a physical property which can be measured to provide a map of the sludge buildup on the outer surfaces of steam generator tubing in pressurized water reactor electrical generating facilities.
Apparatus for the remote detection of magnetic materials can be classified into two groups, depending on whether they are implemented through the use of static or of periodically varying magnetic fields. While the latter group of methods can be made quite sensitive, their use is complicated by an inherent sensitivity to materials of high electrical conductivity such as are employed in steam generator tubing walls, whether these materials are also magnetic or not. The conductive metallic wall between the probe and the deposits therefore forms a source of interference. Since the use of static magnetic fields does not present this problem such probes are desirable in this particular application.
In addition, a probe is desired that is capable of detecting small discontinuities associated with thin films of sludge deposits. Further, in order to supply complete information on the buildup of deposits on the outer surface of steam generator tubing it is desired that such probes be directionally sensitive to the radial distribution of the deposits as well as the longitudinal deposit distribution along the length of the steam generator tubing.